Merge Two Separate Equal Objects to Edit Them at the Same Time
When you select one of the objects, selecting one object as your root object, then you are going to want to select your secondary object to link.
Then by pressing Ctrl + L, it will allow you link the object's data to the main object. That will let you edit the main mesh you selected as the root, meanwhile the other mesh will be edited as well.
But please be mindful that it likely has to have the same topology.

and apply the object data to the mesh and it should allow you to edit both meshes.